The present invention relates to needles for medical applications, for example for insertion into tissue. More particularly, it relates to a needle for insertion of a dialysis membrane into human or animal tissue, for example, a needle with an elongate needle region and an end portion which adjoins the needle region and forms a needle tip for insertion into tissue, wherein the needle carries or has an associated dialysis membrane.
It may be necessary, for a wide variety of reasons, to introduce a medical device or a test device into a tissue, for example a human, animal or other biological tissue. To do this, the surface of the tissue must be penetrated, and, in doing so, the opening made in the tissue, and the force applied to penetrate the surface, should be as small as possible in order to minimize any pain or tissue lesions caused. At the same time, however, the requirements of the medical treatment, application, procedure or test must be able to be satisfied, for which reason large insertion openings and depths of insertion are often needed. To introduce or remove a product, for example a medical, pharmaceutical or cosmetic fluid, or to permit an exchange of a test fluid with the tissue, as is necessary, for example, in dialysis, round hollow needles are used. These hollow needles are beveled at their distal or tissue penetrating end in such a way that an oval ring surface is obtained with a cutting edge forming the tip of the needle. A needle of this kind has been disclosed, in the form of a hypodermic needle, by Therumo Medical Corporation. This needle is additionally beveled in a hyperbola area of the oval surface of the needle tip, so that two bevels are produced which form a sharp tip at the reverse point of the hyperbola.
In microdialysis probes, probe needles are used for conveying a perfusion solution through the tissue, this solution interacting with the tissue environment via a dialysis membrane. Since both a supply line and a discharge line are needed for the perfusion solution, the needles for this reason cannot be made very thin. Instead they generally have a thickness which makes insertion of the needle unpleasant for the user.
The penetration of a needle into a tissue takes place in two stages. First, the surface of the tissue must be penetrated by the tip of the needle. A pressure force which has to be applied in order to do this, and which is referred to hereinafter as the initial penetration force, is lessened by using a fine pointed tip rather than a cutting edge. After the tip of the needle has penetrated into the tissue, the needle region which adjoins the end portion forming the tip penetrates into the tissue. Since this needle region has a thicker cross section, i.e., it is thicker than the end portion of the needle tip, a greater force has to be applied, referred to hereinafter as the complete penetration force, than is necessary for the initial penetration force of the tip. This complete penetration force is lessened by using a cutting edge in contrast to a pointed tip at the needle tip. When configuring the tip area of the needle, a compromise must therefore be found between a fine pointed tip and a cutting edge.
In conventional needles for medical purposes, this requirement has received little attention because needle tips allow little leeway when it comes to their design. For example, tubular needles with a tube or even two tubes for a supply line and a discharge line can be designed in different tip shapes only with difficulty. Burrs often form at the edges, these burrs increasing the force that has to be applied and thus making the penetration of the needle particularly unpleasant for the user. Moreover, in conventional needles, the tip or edge areas of the needle tip often bend when the needle tip is inserted, as a result of which the force to be applied for penetration is further heightened. In particular, needles of microdialysis probes often cannot be made especially pointed or sharp since, in many cases, the dialysis membrane is arranged in or around the needle tip area and could be damaged by sharp edges or points.